Hi Pekka, I think you meant translucent in the first and last post -- not opaque, hence the confusion.
eskoilola, re. "Do You actually believe that a small wooden sword would have any chances of survival in the hands of a viking ?"
Depends on the viking (or in this case samurai, not to mix up our cultures) and the type of prop use. If for costume show primarily, wood will certainly do. But if the proposal is for actual clashing with replica-looking blades, any hard plastic will also dent pretty bad and even break. Light weight hollow soft plastic like polypropylene would be more appropriate and safer.
And btw to go along with Japanese culture, bamboo swords are quite durable, even in practice battles. Not that they are particularly safe if of scale proportions. Tubular blades are much safer.
Pekka while you may not prefer to use plaster of Paris for other reasons, it can be reinforced just like any modern plastic resin would need to be -- and you can use easily obtained or scrap wire or screen or mesh, as well as set into a wooden box for support. It cures very quickly, and no, it does not shrink, which is why it is traditional for molding. Vaseline (petroleum jelly) is a great release for it, all easily obtained and inexpensive materials.
But of course everything we do must appeal to us, and if epoxies and glass fiber are what you want to work with, that's the way to go. I'm jus throwing out possibilities to consider.